Power supplies have a limited minimum size that such electronic systems can attain, relying as they do on relatively large transformers with relatively large ferrite cores and magnet wire windings. Planar transformers ease this limitation and allow designers to achieve the low profiles required for circuit board mounting in space constrained applications. Connections to an outside circuit, such as the power semiconductors, are made by standard circuit board pins.
FIG. 1 shows a standard transformer 100. The transformer 100 comprises a winding spool 110. The winding spool 110 is configured to allow wire or cable (not shown) to be wound about the winding 110. Generally, there are at least two independent wires or cables wound about the spool 110 to effectuate the forming of secondary voltages from a primary voltage. It is generally known to those of ordinary skill that applying an alternating current voltage to a primary winding will generate an alternating current voltage on a secondary winding. A ratio between the number of turns of the primary winding and the number of turns of the secondary winding determines the ratio of amplitude between the signal applied to the primary and the signal measured from the secondary. Furthermore, multiple primary and secondary windings are generally employed for greater efficiency. The winding is mounted about a magnetic core 120 with extended sections 130. In some embodiments, a cap 140 is utilized to cover the transformer 100. Inputs and outputs 150 are electrically coupled to the primary and secondary windings to couple input and output signals from the transformer 100 to the outside world.
FIG. 2 shows the substrate layers 201-205 of a planar transformer. Although a planar transformer operates on the same basic principles as a standard transformer, its construction is different. Rather than wires around a core as described above for a standard transformer, these substrate layers have disposed thereupon copper traces 206 in a circular fashion about an opening 210. These traces perform essentially the same function as the wires in the standard transformer. When a primary voltage signal is applied to one set of inputs 211 that are electrically coupled to one set of copper traces 206, secondary voltage signals are formed at the outputs 215. The ratio of amplitude between the input and output is set by number of times the copper is wound about the opening. The substrates 201-205 are able to be any material that is convenient for mounting copper thereupon. In some embodiments, the substrate is a material such as FR4, a standard material in making circuit boards. Rather than mounting copper thereupon, pre-plated copper is able to be etched away by standard etching techniques.
FIG. 3 shows an exploded diagram of a standard planar transformer 300. In this exemplary embodiment, a core includes a top core 310, a central core 315 integrally formed thereupon and a bottom core 360. Alternatively, the central core 315 is able to be welded on or attached by another convenient means. The central core 315 is configured and properly sized to fit through an opening 320 in the laminate body 330 on which the copper traces (not shown) are disposed. A voltage is applied to a set of primary inputs 340. As mentioned above, the voltage signal causes the formation of various output signals based on the ratio of the number of turns between the primary and secondary windings. The planar transformer 300 is able to have at least one primary input 340 and at least one secondary output 350. The top core 310 is magnetically coupled to a bottom core 360. In this example, the inputs 340 and outputs 350 are in the form of through-hole pins. Alternatively, surface mount pads are able to replace the through hole pins.
However, given the compact size and planar configuration, planar transformers are often tightly packed into an area and come into thermal contact with other circuits, and the like. In such high temperature environments, it is important that the planar transformer have a thermal management system to prevent overheating and to enable cooling. Simply mounting a heat sink element to a planar transformer may not be satisfactory. The thermal performance of a mounted heat sink can be inadequate. Furthermore, the addition of a heat sink increases the number of steps to manufacture a system that has a planar transformer and will increase the cost of manufacturing such a device.
What is needed is a planar transformer that has enhanced heat transfer efficiency. What is also needed is a planar transformer that is easy to manufacture. What is additionally needed is a planar transformer that both has enhanced heat transfer efficiency and adds no additional manufacturing steps.